The photoconductive process in electrophotographic sensitive materials consists of (1) a step of generating electric charges by exposure, and (2) a step of transferring the electric charges.
An example of carrying out the steps (1) and (2) by means of a single substance includes a selenium plate, which is well known. On the other hand, an example of carrying out the steps (1) and (2) by means of different substances, respectively, include a combination of amorphous selenium and poly-N-vinylcarbazole, which is also well known. The process of carrying out the steps (1) and (2) by means of different substances, respectively, has advantages such as that the choices of selecting materials used for the electrophotographic sensitive materials is expanded, by which electrophotographic properties, such as sensitivity and charge acceptance, etc., of the sensitive materials are improved and that materials suitable for production of a coating layer in the electrophotographic sensitive material can be selected from a broad range.
Hitherto, inorganic substances such as selenium, cadmium sulfide, or zinc oxide, etc., have been used as the photoconductive material in the sensitive material used in electrophotographic processes.
As has been disclosed in U.S. Pat. No. 2,297,691, by Carlson, in the electrophotographic process, a photoconductive material comprising a base coated with a substance which is insulator in the dark, electric resistance of which varies corresponding to exposure by imagewise exposing to light, is used. This photoconductive material is generally electrically charged in the dark after being subjected to dark adaptation for a suitable period of time, This material is then imagewise exposed to light in the form of a radiation pattern which has the effect of reducing surface electric charges corresponding to the relative energy included in the radiation pattern. The surface electric charges or electrostatic latent images remaining on the surface of the photoconductive layer (photosensitive layer) are then brought into contact with a suitable electroscopic indication material, namely, a toner, to form visible images. The toner is allowed to adhere to the surface of the sensitive layer corresponding to the electric charge pattern, whether the toner is contained in an insulating liquid or in a dry carrier. The incidation material adhered on the surface can be fixed by known means such as by heat, pressure, or a vapor of a solvent. Further, the electrostatic latent images can be transcribed on a second base (for example, paper, film, etc.). Likewise, it is possible to develop the electrostatic latent images transcribed onto the second base.
Basic characteristics required for the electrophotographic sensitive materials used in such an electrophotograhic process include the following: (1) the sensitive material can be electrically charged in the dark so as to have a suitable electric potential, (2) the degree of disappearance of electric charges is small in the dark, and (3) the electric charges can be rapidly dispersed by light exposure. However the above described inorganic substances used hitherto have various drawbacks at the same time they have a lot of advantages. For example, selenium, which is widely used at the present time, sufficiently satisfies the above described requirements (1) through (3). However, it has drawbacks in that the cost of production is high because of the severe conditions required for production, in that it is difficult to form into a belt because of lack of elasticity, and that it is necessary to pay attention to handle because it is sensitive to heat and mechanical shock. Cadmium sulfide and zinc oxide have been used as a sensitive material by dispersing in a resin binder, but they can not be repeatedly used, because they have mechanical drawbacks with respect to smoothness, hardness, tensile strength, anti-friction properties, etc.
In the recent years, various electrophotographic sensitive materials using organic substances have been proposed, in order to overcome the drawbacks in the inorganic substances, and some of them have been put to practical use. Examples include a electrophotographic sensitive material comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluorene-9-one (U.S. Pat. No. 3,484,237), a sensitive material composed of poly-N-vinylcarbazole sensitized with a pyrilium dyestuff (Japanese patent publication No. 25658/73), a sensitive material comprising an organic pigment as a main component (Japanese patent application (OPI) No. 37543/72) and a sensitive material comprising an eutectic complex composed of a dye and a resin (Japanese patent application (OPI) No. 10735/72). Although these sensitive materials are believed to have a high practical value because of having excellent characteristics, the fact is that there is not any organic substance which is simply produced, has sufficient electrophotographic characteristics, and sufficiently satisfies requirements for an electrophotograhic process, considering the various requirements for the electrophotographic sensitive materials, such as good wave selectivity required for applying the sensitive material as a laser beam printer or an indication element.
As a result of carrying out studies about the charge generating materials, it has now been found, as will be described in more detail below, that barbituric acid derivatives and thiobarbituric acid derivatives represented by the formulas (I) and (II) described below, and hereinafter referred to as (thio)barbituric acid derivative, are excellent as charge generating materials and sufficiently satisfy all requirements for the electrophotograhic sensitive materials.
Merocyanine dyes having a barbituric acid nucleus or a thiobarbituric acid nucleus have been known as spectral sensitizing dyes for silver salt sensitive materials, and many studies on them have been done in that field. In the recent years, it has been attempted to use these merocyanine dyes as electrophotographic sensitive materials, and, particularly, as electric photosensitive particles for an electrophoretic image forming process, but satisfactory characteristics have not been shown.
An example of using (thio)barbituric acid derivatives as the electrophotographic material has been disclosed in U.S. Pat. No. 3,536,484. However, thiobarbituric acid derivatives used in this U.S. Patent have drawbacks in that they have inferior stability to light, heat, and air oxidation, because a substituted phenyl group is linked to a thioburbituric acid residue through a pentamethine chain having a ring structure, in that synthesis thereof is complicated, and in that they have inferior solubility in organic solvents.